1. Field of the Invention
The present invention relates to the structure of an audio decode apparatus receiving a stream of audio digital data recorded digitally or transmitted as a digital signal to decode the same into a digital signal that can be converted into an analog audio signal.
2. Description of the Background Art
In accordance with the progress of the recent digital information compression technique, digital information is recorded/reproduced according to a standard such as the MPEG 2 (Moving Picture Experts Group 2) as the standard of coding audio and motion pictures (video), for example, into digital signals for multiplexing into audio video signals.
The coding system of a digital audio signal is not limited to the aforementioned MPEG audio. For example, the coding system of AC-3 (Audio Coding-3) employed in the DVD (Digital Video Disc) is known.
The decode apparatus of the systems corresponding to the above-described coding methods commonly decode a stream of audio digital data such as the MPEG and AC-3 and also generate an audio signal converted into a digital signal according to pulse code modulation (referred to as PCM hereinafter). In the following, these digital audio signals are referred to as "PCM audio signal", and audio information converted into such PCM audio signals is referred to as "PCM audio data".
FIG. 12 is a schematic block diagram showing a structure of a conventional audio decoder 5000.
Conventional audio decoder 5000 includes an audio signal decode unit 5011 receiving audio sample data read out from a recording medium of digital audio signals such as a DVD to apply a decoding process, and an audio signal converter unit 5013 receiving the signal from audio signal decode unit 5011 via a data buffer 5012 to convert the same into an audio digital signal that can be converted into an analog audio signal.
In many cases, audio signal decode unit 5011 is formed of a digital signal processor (referred to as DSP hereinafter) or a S/W signal processing block that carries out a decoding process by software such as a microcomputer.
For example, when the audio sample data is a multi-channel signal, the time series data (serial data) output from audio decode unit 5011 is provided to a plurality of digital-analog converters (referred to as "D/A converter" hereinafter) corresponding to each channel by audio signal converter unit 5013.
FIG. 13 is a schematic block diagram showing a structure of audio sample data output from audio signal decode unit 5011.
In FIG. 13, time is plotted along the horizontal direction.
More specifically, audio sample data output from audio signal decode unit 5011 corresponds to the continuation of data of 6 channels as time series data. Data 21a-21f correspond to channels 1-6, respectively, followed by sample data 21g-21k corresponding to channels 1-5, respectively.
Data corresponding to the data of each channel is referred to as PCM audio data 32 hereinafter.
Examples of such multichannel audio data include the AC-3 multichannel signal, i.e., data formed of audio data of 5 channels and low frequency effect audio (LFE).
When PCM audio data (audio sample data) which is the time series data shown in FIG. 13 is decode-processed by audio signal decode unit 5011 and then applied to audio signal converter unit 5013 directly as time series data, problems set forth in the following will be encountered.
In a CD and the like which is the recording medium of conventional digital audio signals, the sampling frequency fs of each audio sample data is 48 kHz. The quantizing accuracy thereof is 16 bits.
In the DVD specification, the sampling frequency fs is 96 kHz or 192 kHz, in addition to the aforementioned 48 kHz.
As to the quantizing accuracy, 20 bits or 24 bits are employed in addition to the aforementioned 16 bits.
Some DVD specifications have copy protection specified with respect to the audio sample data. In this case, the audio signal decode unit may have to carry out the decoding process with the sampling frequency fs forced to 48 kHz and the quantizing accuracy forced to 16 bits.
As to music information recorded on a DVD, for example, some have the former part of the melody not subjected to copy protection, and the latter part of the melody subjected to copy protection.
Furthermore, if attribute information such as application of copy protection is not transferred together with the PCM audio data applied to audio signal converter unit 5013, the digital signal output from the audio signal converter unit may not be able to correspond to the dynamic change, if any, in the attribute of the audio data.
When error occurs and resynchronization is to be applied after initiating audio sample data transfer, there was a problem that audio decode unit 5011, buffer 5012 and audio converter unit 5013 must be initialized to restart the transfer.
This problem in resynchronization will be described in further detail.
FIG. 14 is a schematic diagram to describe the structure of a program stream which is the signal read out from a digital audio video recording medium such as a DVD.
For example, a presentation time stamp (referred to as PTS hereinafter) indicating the time when the audio elementary stream is to be reproduced as the system clock time and an audio elementary stream or video elementary stream corresponding to the PTS are included in time series in the program stream.
The information amount of video signals is generally considerably greater than the information amount of audio signals. Therefore, the audio elementary stream is included intermittently in the program stream.
The audio elementary stream is further divided into various frame data. Each frame is formed of a header and audio data.
The audio data includes a plurality of the above-described audio sample data.
If audio decode apparatus 5000 cannot apply resynchronization without initialization of the system when error occurs during the reproduction of the audio data according to the structure shown in FIG. 14, there is a problem that the deviating state in the reproduction timing between the audio information and the video information or discontinuation in the audio will continue for a long period of time.